Chemical formulas, such as H2O (water) and CO2 (carbon dioxide), are handy as they identify which atoms are present in a chemical compound. This can in turn help to make predictions about their properties. But only knowing which atoms make up a compound sometimes isn’t enough. It helps to know how the atoms are arranged.
Many substances, including H2O and CO2, are found as molecules. We can’t see individual molecules because they are far too small. However, molecules do have shapes. For example, H2O and CO2 are both made of three atoms. The atoms in H2O are arranged in a V-shape, with the oxygen atom at the centre, while in CO2 they form a straight line, with the carbon atom at the centre.
Imagine a line that cuts the V of the H2O molecule in half. Imagine a similar line, perpendicular to the CO2 molecule that passes through carbon atom, cutting it in half. You will see that the halves are mirror images of each other. Both H2O and CO2 are symmetrical. However, not all molecules have a symmetrical structure. Many larger, more complicated molecules are asymmetrical.
The symmetry of a molecule can be used to make predictions about its properties. One such property is whether or not a molecule will absorb infrared radiation. All molecules that are asymmetrical will absorb infrared radiation. However, some symmetric molecules, including H2O and CO2, absorb as well. Why?
Think of a molecule as balls (the atoms) connected by springs (the chemical bonds). The atoms don’t sit still; they vibrate, and their bonds are constantly bending and stretching.
Take the linear CO2 molecule. Now imagine that one of the oxygen atoms stretches away from the carbon atom, while the other stretches towards it. The molecule is no longer exactly the same on both sides of the original centre line – the symmetry is broken. A similar thing happens with the H2O molecule.
On the other hand, an oxygen molecule consists of two oxygen atoms bonded together (O2). No matter which way the bonds stretch and bend, the O2 molecule is always symmetric, and O2 doesn’t absorb infrared radiation.
So while asymmetric molecules all have asymmetric vibrations, some symmetric molecules have vibrations that break their symmetry as well.